The invention relates to a developing device having a toner carrying body which causes toner particles to adhere to a surface on which a latent electrostatic image has been formed (e.g., a surface on a photosensitive drum of an electronic copying machine) and a method of fabricating such toner carrying body.
A developing device using a toner carrying body of this type is disclosed in Japanese Patent Unexamined Publication No. 223769/1986 and the like. The developing device will be outlined below with reference to FIGS. 4 to 7A-7C.
A fix/support shaft 01 shown in FIG. 4 is fixed and supported by a not shown support member, and fixes and supports a cylindrical magnet roll MR0. As shown in FIG. 5, the magnet roll MR0 has a plurality of magnetic poles arranged inwardly around its outer peripheral surface, the poles extending along its length. At both end portions of the fix/support shaft 01 are a pair of rotating support members 02, 02 rotatably supported, and both ends of a cylindrical toner carrying body 03 are fixed and supported by these rotating support members 02, 02. The rotating force of a not shown drive motor is transmitted to the toner carrying body 03 through gears, the support members 02, and the like. The toner carrying body 03 also has an electrically conductive inner layer 03a and a semiconductive outer layer 03b. The layer 03a confronts the magnet roll MR0 while keeping a predetermined distance from the outer peripheral surface of the magnet roll MR0, and the layer 03b is disposed on the outer peripheral surface of the layer 03a in a laminated form. The reason why the outer layer 03b is made semiconductive is that if the outer layer 03b were electrically conductive, discharge would be caused between the toner carrying body 03 and the surface of a photosensitive body when a high voltage is applied therebetween during a development process, and this discharge would act to remove electric charges generated at the photosensitive body surface, producing defects such as white points in a solid black portion of an image or black points in a nonimage portion.
FIG. 5 is a diagram schematically illustrating a developing device using the toner carrying body 03. A hopper 05 which contains an electrically insulating single-component magnetic toner 04 has a toner carrying body insertion inlet 05a formed, and an upper edge of the toner carrying body insertion inlet 05a coincides with an edge of a magnetic blade 05b. This magnetic blade 05b serves to reduce excessive single-component magnetic toner particles so that the thickness of a toner layer formed on the surface of the toner carrying body 03 can be maintained constant.
The toner carrying body 03 is installed in such a manner that a part of it is received by the hopper 05 from the toner carrying body insertion inlet 05a and that the rest is exposed from the hopper 05. The photosensitive body surface 06a of a photosensitive drum 06 is arranged so as to confront the portion of the semiconductive outer layer 03b of the toner carrying body 03 which is exposed from the hopper 05. An electrically conductive layer 06b in the interior of the photosensitive body surface 06a is grounded, while the electrically conductive inner layer 03a of the toner carrying body 03 is grounded through an ac power supply 07 and a dc power supply 08 for biasing, both connected in series.
As shown in FIGS. 6A and 7A, a bias voltage of the dc power supply 08 is set to such a value that a bias potential V.sub.A at the semiconductive outer layer 03b that is determined by such bias voltage becomes a value that is in between a charging potential V.sub.B at an image portion (a black portion) (hereinafter referred to as "image potential") and a charging potential V.sub.L at a nonimage portion (a white portion) (hereinafter referred to as "background potential") on the photosensitive body surface 06a. The ac power supply 07 is set to such a value that a maximum potential V.sub.MAX at the semiconductive outer layer 03b is higher than the image potential V.sub.H at the photosensitive body surface 06a and that a minimum potential V.sub.MIN is lower than the background potential V.sub.L. Therefore, as shown in FIG. 6A, the image potential VH at the photosensitive body surface 06a is higher than the surface potential at the semiconductive outer layer 03b of the toner carrying body 03 during time intervals t.sub.1, t.sub.3, ..., T.sub.2n-l (n=1, 2, ...), and the direction of the electric field between the surface 06a and the layer 03b runs from the semiconductive outer layer 03b to the photosensitive body surface 06a as indicated by arrows Ef' in FIG. 6A. Also, the image potential VH at the photosensitive body surface 06a is lower than the surface potential at the semiconductive outer layer 03b during time intervals t.sub.2, t.sub.4, ..., t.sub.2n (n=1, 2, ...), and the direction of an electric field between the surface 06a and the layer 03b runs from the photosensitive body surface 06a to the semiconductive outer layer 03b as indicated by arrows Er' in FIG. 6A. On the other hand, as shown in FIG. 7A, the background potential VL at the photosensitive body surface 06a is higher than the surface potential at the semiconductive outer layer 03b of the toner carrying body 03 during times t.sub.1, t.sub.3, ..., t.sub.2n-l' (n=1, 2, ...), and the direction of the electric field between the surface 06a and the layer 03b at these time intervals is as indicated by arrows Ef' in FIG. 7A, while the background potential V.sub.L is lower during time intervals t.sub.2', t.sub.4', ..., t.sub.2n' (n =1, 2, ...), and the direction of the electric field is as indicated (a) by arrows Er'.
As described before, the relative magnitudes of the image potential V.sub.H or background potential V.sub.L at the photosensitive body surface 06a and of the surface potential at the semiconductive outer layer 03b of the toner carrying body 03 get inverted in correspondence with the cycle of the ac power supply 07, and this generates an electric field whose direction is changed alternately between the photosensitive body surface 06a and the semiconductive outer layer 03b. Assuming that an electric field generated when the potential at the photosensitive body surface 06a is higher than that of the semiconductive outer layer 03b (hereinafter referred to as "toner adhering electric field") is designated by Ef and that an electric field generated when the potential at the photosensitive body surface 06a is lower than that of the semiconductive outer layer 03b (hereinafter referred as "toner detaching electric field") is designated by Er, each electric field Ef or Er becomes more intensive with a shorter distance between the photosensitive body surface 06a and the semiconductive outer layer 03b as shown in FIGS. 6B, 6C, 7B, 7C. And the intensity of each electric field Ef or Er in regions A2, A2' gets larger than that in regions A1, A1', and A3, A3'.
If the toner carrying body 03 and the photosensitive drum 06 are rotated in directions of arrows X and Y, respectively, when this type of developing device is operated, then the single-component magnetic toner 04 is, e.g., negatively charged by friction, adheres to the semiconductive outer layer 03b of the toner carrying body 03 to a predetermined thickness adjusted by the magnetic blade 02b, and approaches the photosensitive body surface 06a.
At this instance, the single-component magnetic toner 04 on the semiconductive outer layer 03b behaves differently between a case where the potential at the photosensitive body surface 06a is equal to the image potential V.sub.H (as shown in FIGS. 6A to 6C) and a case where it is equal to the background potential V.sub.L (as shown in FIGS. 7A to 7C). The case where the surface potential at the photosensitive body surface 06a is equal to the image potential V.sub.H will be described first.
In FIGS. 6A to 6C, when the semiconductive outer layer 03b approaches the photosensitive body surface 06a to enter the region A1, the toner adhering electric field Ef between both surfaces 03b and 06a becomes larger than a movement start threshold electric field Efth (see FIG. 6C), the electric field Efth being an electric field which causes the single-component magnetic toner 04 to move from the semiconductive outer layer 03b to the photosensitive body surface 06a (by flying in air), while the toner detaching electric field Er remains at its movement start threshold electric field Erth or less. At this instance, the single-component magnetic toner 04 which has adhered to the semiconductive outer layer 03b moves to the photosensitive body surface 06a to adhere thereto. A term "adhering movement" which will hereinafter be used is intended to mean movement of the toner 04 from the semiconductive outer layer 03b to the photosensitive body surface 06a, and a term "detaching movement", movement of the toner 04 opposite to the adhering movement. The former will be indicated by an arrow F (see FIGS. 6C, 7C) and the latter by an arrow R (see FIGS. 6C, 7C).
As the semiconductive outer layer 03b approaches the photosensitive body surface 06a further to enter the region A2, the toner detaching electric field Er between both surfaces 03b and 06a also becomes larger than its movement start threshold electric field Erth that causes the single-component magnetic toner 04 to start moving from the photosensitive body surface 06a to the semiconductive outer layer 03b. As a result, the electric fields Ef, Er generated by the ac power supply 07, with their direction being alternated, cause the single-component magnetic toner 04 to shuttle between the semiconductive outer layer 03b and the photosensitive body surface 06a with its adhering movement F from the layer 03b to the surface 06a and its detaching movement R opposite to the adhering movement F. At this instance, since the toner adhering electric field Ef is larger than the toner detaching electric field Er, the toner adhering movement F has greater power than the toner detaching movement R.
Then, when the semiconductive outer layer 03b and the photosensitive body surface 06a rotate further to enter the region A3, the toner adhering electric field Ef between both surfaces 03b and 06a remains at the movement start threshold electric field Efth or more, while the toner detaching electric field Er becomes the movement start threshold electric field Efth or less. As a result, the single-component magnetic toner 04 moves only from the semiconductive outer layer 03b of the toner carrying body 03 to the photosensitive body surface 06a. After both surfaces 03b and 06a have passed the region A3, the movement of the single-component magnetic toner 04 is stopped. Accordingly, the single-component magnetic toner 04 adheres on the photosensitive body surface 06a that is held at the image potential V.sub.H.
The case where the surface potential at the photosensitive body surface 06a is equal to the background potential will be described next.
In FIGS. 7A to 7C, when the semiconductive outer layer 03b approaches the photosensitive body surface 06a to enter the region A1, the toner detaching electric field Er between both surfaces 03b and 06a becomes larger than the movement start threshold electric field Erth, while the toner adhering electric field Ef remains at the adhering movement start threshold electric field Efth or less. At this instance, if some single-component magnetic toner 04 were present on the photosensitive body surface 06a, such single-component magnetic toner 04 should move toward the semiconductive outer layer 03b. However, with no such single-component magnetic toner 04 that should make a detaching movement R present on the photosensitive body surface 06a, there occurs no movement of the single-component magnetic toner 04.
As the semiconductive outer layer 03b approaches the photosensitive body surface 06a further to enter the region A2, both the toner detaching electric field Er and the toner adhering electric field Ef between both surfaces 03b and 06a become larger than the movement start threshold electric fields Erth and Efth, respectively. As a result, the electric fields Ef, Er generated by the ac power supply 07, with their direction being alternated, causes the single-component magnetic toner 04 to shuttle between the semiconductive outer layer 03b and the photosensitive body surface 06a with its adhering movement F from the layer 03b to the surface 06a and its detaching movement R opposite to the adhering movement F. At this instance, since the toner detaching electric field Er is larger than the toner adhering electric field Ef, the toner detaching movement R has greater power than the toner adhering movement F.
Then, when the semiconductive outer layer 03b and the photosensitive body surface 06a rotate further to enter the region A3, the toner detaching electric field Er between both surfaces 03b and 06a remains at the movement start threshold electric field Erth or more, while the toner adhering electric field Ef becomes the movement start threshold electric field Efth or less. As a result, the single-component magnetic toner 04 moves only from the photosensitive body surface 06a to the semiconductive outer layer 03b. After both surfaces 03b and 06a have passed the region A3, the movement of the single-component magnetic toner 04 is stopped. Accordingly, the single-component magnetic toner 04 no longer adheres on the photosensitive body surface 06a that is held at the background potential V.sub.L.
The development method described with reference to FIGS. 6A to 6C and 7A to 7C comprises the steps of: causing a single-component magnetic toner to adhere to the toner carrying body 03 surface by the magnetic force of the magnet roll MR0; forming the adhering toner into a layer of a predetermined thickness by the magnetic blade 05b; moving the thus processed toner to the photosensitive body surface 06a before development. A development method similarly involving movement of a toner may also be applied to a single-component nonmagnetic toner.
To cause the single-component nonmagnetic toner to adhere to the toner carrying body surface forces such as electrostatic forces (mirror image forces), adhering forces, and van der waals forces are used since no magnetic force can be utilized. Since the adhesiveness for causing the toner to adhere to the toner carrying body surface possessed by these forces is small compared with the magnetic force, various design considerations are made to form an even toner layer on the toner carrying body surface using these forces whose adhesiveness is small. For example, in a conventional developing device using a single-component nonmagnetic toner as shown in FIG. 8, a toner supply member 011 for causing toner particles to adhere to the toner carrying body 010 surface is employed, and special considerations are given to the shape and position of a thickness regulating member 012. In FIG. 8, the thickness regulating member 012 includes a stainless strip 013 and a silicone rubber 014 which is adhesively fixed at a free end of the strip 013. As the toner carrying body 010 surface rotates, the single-component nonmagnetic toner is fed to a nipped portion between the toner carrying body 010 surface and the thickness regulating member 012 that is nipped (in pressure contact) with the toner carrying body 010 surface. The single-component nonmagnetic toner that has reached the nipped portion is then charged by friction, adsorbed by the electrostatic force or the like onto the toner carrying body 010 surface and levelled by the thickness regulating member 012 that is nipped with the toner carrying body 010 surface to be formed into an even layer.
By the way, developing devices generally produce nonuniform densities and the like in developed images once nonuniformity is caused in the distribution of toner particles on the toner carrying body surface. Therefore, the toner carrying body used in the developing devices must be arranged so that the toner can adhere to its surface evenly.
Further,, since inconsistency in the distance between the toner carrying body surface and the photosensitive body surface causes nonuniformity in developed images, the distance must be maintained at a constant value.
From the above requirements, a toner carrying body which is to be applied to a developing device using a single-component nonmagnetic toner must possess the following properties and characteristics.
(a) The resistivity by volume of a material that is used to form the semiconductive outer layer is less erratic.
(b) The outer diameter of the semiconductive outer layer is less erratic and its surface is macrographically smooth.
(c) The amount of flexion of the toner carrying body shaft is small.
Further, a toner carrying body to be applied to a developing device using a single-component nonmagnetic toner whose adhesiveness to the toner carrying body is extremely small requires the following additional characteristic in order to increase its adhesiveness.
(d) Uniform irregularities micrographically having a surface roughness Rz of from 1.0 .mu.m to 10 .mu.m is formed on the
When the surface roughness Rz is less than 2.5 .mu.m, defects such as white points are accidentally produced in a solid black portion of an image due to lack of supplying quantity of the toner. In addition, when any foreign matter (undesirable dust and the like) is accidentally introduced into the gap between the toner carrying body and the magnetic blade, it is difficult to remove it.
When the surface roughness Rz is more than 4.5 .mu.m, the toner which is adhered to the photosensitive drum is not correctly transferred to the latent image due to lack of electrification of the toner. Further, there is a problem that the poor electrified toner is accidentally transferred to the back ground, that is the portion except for the latent image portion.
Furthermore, a toner carrying body to be applied to a developing device using a single-component magnetic toner may provide additional advantages if it has the following characteristic.
(e) If a magnet roll producing a small magnetic force is used, the distance between the toner carrying body surface and the magnet roll surface is as small as possible to maximize the magnetic force of the toner carrying body surface.
By the way, the method of fabricating the conventional toner carrying body 03 shown in FIGS. 6A to 6C forms a cylindrical semiconductive resin sleeve by means of compression molding, injection molding, or extrusion molding using a material such as phenol resin having an electrically conductive material mixed therewith. And with this cylindrical sleeve, a semiconductive outer layer 03b is formed on the sleeve, and an electrically conductive inner layer, inside the sleeve. The method, being as such, addresses the following problems.
(1) Unsatisfactory dimensional accuracy in the molding requires that the formed body be subjected to post-processing such as grinding of the outer diameter and processing of both ends, thereby increasing the number of processes involved.
(2) The forming of the electrically conductive layer on the inner peripheral surface of the cylindrically formed semiconductive resin sleeve is cumbersome compared with the formation of layers on the exposed outer peripheral surface.
(3) The semiconductive resin sleeve is made of phenol resin having an electrically conductive material mixed therewith and the like, and the rigidity of the resin is not adequately large. Therefore, the semiconductive resin sleeve cannot be made thinner, and if a single-component magnetic toner is used, the distance between the semiconductive resin sleeve surface, i.e., the toner carrying body 03 surface, and the magnet roll MR0 surface may not be reduced. Thus, a magnet roll MR0 whose magnetic force is larger must be used to increase the magnetic force of the toner carrying body 03 surface.
(4) The above problems (1) to (3) contribute to elevating the fabrication cost of the toner carrying body 03 shown in FIG. 4.
Also conventionally known is a toner carrying body which adhesively combines a resin-made semiconductive sleeve with the outer surface of a metal-made electrically conductive sleeve using an electrically conductive adhesive. However, in this type of toner carrying body, failure to uniformly apply the electrically conductive adhesive to a gap between both sleeves may cause irregular recesses to be created on the toner carrying body surface or make surface potentials inconsistent due to defective adhesion. Since both sleeves cannot be made thinner, the distance between the magnet roll surface and the toner carrying body surface should become large when the magnet roll is installed inside the sleeves. This reduces the magnetic force of the toner carrying body surface even if the magnetic force of the magnet roll is large.
Further, Japanese Patent Examined Publication No. 5704/1089 discloses a toner carrying body and a developing device using such toner carrying body. This toner carrying body has a metallic sleeve surface and a resin layer coated with a compound of silane on that surface. However, the art disclosed in this publication does not consider a case of using a single-component nonmagnetic toner with small adhesiveness to the toner carrying body, nor does it consider the profile of the toner carrying body surface. Therefore, the toner carrying body proposed in the above publication has the problem of not being applicable to a developing device using a single-component nonmagnetic toner that is less adhesive.
Still further, Japanese Patent Examined Publication No. 23864/1990 proposes a toner carrying body, which is formed by grinding a dielectric layer coated on a metallic sleeve surface, forming on the ground surface an electrically conductive particle layer coated with a resin through an adhesive, and grounding the thus formed surface so that the electrically conductive particles are exposed. However, the construction and fabrication method of the toner carrying body proposed in this publication is disadvantageously complicated.